The present invention relates to a method for the diagnosis of an exhaust gas purification system for an internal combustion engine.
As is known, internal combustion engines that are currently in use are provided with exhaust gas purification systems that are intended to minimise emissions of polluting substances, for instance nitrogen oxides, carbon monoxide and non-combusted hydrocarbons.
Some parts of these systems are subject to wear and it is therefore necessary, in order to ensure removal of the polluting emissions of an acceptably efficient level, that their operating condition is constantly monitored.
For greater clarity, an exhaust gas purification system of the type commonly used is shown by reference numeral 1 in FIG. 1.
The system 1 is connected to an internal combustion engine 2 and comprises a pre-catalyst 3, disposed along an exhaust duct 4 to the rear of the engine 2, and a main catalyst 5, disposed further downstream. The pre-catalyst 3 is typically a three-way catalyst or TWC.
A first oxygen sensor 6, typically a linear LAMBDA or UEGO probe, is disposed at the input of the pre-catalyst 3, while a second oxygen sensor 7, for instance a probe of LAMBDA ON/OFF type, is disposed at the output of the main catalyst 5. An output temperature sensor 8 also measures the temperature of the exhaust gases at the output of the pre-catalyst 3.
The oxygen sensors 6 and 7 generate a first and respectively a second exhaust gas composition signal V1 and V2, representative of titres xcexI and xcexF at the input and output of the pre-catalyst 3, while the temperature sensor 8 supplies an output temperature signal Vo representative of the output temperature To of the exhaust gases on output from the pre-catalyst 3. These signals are supplied as input to a control unit 10 that further receives a plurality of engine-related parameters (for instance number of revolutions RPM and load L), on the basis of which the operating point of the engine 2 can be determined. The control unit 10 performs a diagnostic algorithm and generates as output an error signal E, for instance of logic type, which assumes a first logic value when a malfunction of the system 1 is detected.
The pre-catalyst 3 and the main catalyst 5 are the most important components of the system 1 and it is therefore fundamental to ensure that their efficiency is constantly kept above a minimum threshold. It is therefore necessary to provide methods that make it possible to ascertain whether, as a result of ageing or malfunction, there has been any deterioration of their performance and whether maintenance work needs to be carried out to ensure that the system is returned to normal operation.
Methods are in particular known for the diagnosis of exhaust gas purification systems, substantially similar to the system 1 described above, based on the information supplied to the control unit 10 by the oxygen sensors 6 and 7.
These methods have some drawbacks.
In particular, the fact that the known methods make it possible to carry out only an overall diagnosis of the system, while evaluation of the performance of the individual pre-catalyst 3 is difficult and not very reliable, is a drawback.
Diagnosis of the pre-catalyst 3 is possible only during ignition transients of the engine 2, since the main catalyst 5 requires longer times to achieve maximum efficiency. Therefore, in the initial phases of operation, during which the pre-catalyst 3 and the main catalyst 5 are being brought to respective operating temperatures, the removal of the polluting emissions may be attributed completely to the action of the pre-catalyst 3 which reaches conditions of maximum efficiency more rapidly.
It is not possible, however, to determine the duration of the heating transient of the pre-catalyst 3 or the moment at which the main catalyst 5 starts to play a significant part in the removal of the polluting emissions with any certainty.
It is known, moreover, that the duration of the heating transient of the pre-catalyst 3, increases, in a manner that cannot be accurately determined, with the ageing of this pre-catalyst 3. Consequently, when the efficiency of the system 1 decreases because of wear, the traditional diagnostic methods become less reliable.
The object of the present invention is to provide a diagnostic method which is free from the drawbacks described above and is, in particular, reliable and not influenced by the ageing of the pre-catalyst.
The present invention therefore relates to a method for the diagnosis of an exhaust gas purification system in an internal combustion engine, comprising a pre-catalyst and a main catalyst disposed along an exhaust duct of an internal combustion engine, first temperature detection means disposed at the input of the pre-catalyst and generating a first temperature signal representative of an input temperature TI of the exhaust gases at the input of this pre-catalyst, second temperature detection means disposed at the output of the pre-catalyst and generating a second temperature signal representative of an output temperature To of the exhaust gases at the output of this pre-catalyst, oxygen sensor means, disposed at the input of the pre-catalyst and generating an exhaust gas composition signal, representative of an input oxygen titre xcexI, a control unit receiving as input the first and second temperature signals and the exhaust gas composition signal and supplying as output an error signal, characterised in that it comprises the stages of:
a) determining a real heat QR supplied to the exhaust gases within the pre-catalyst as a function of the input temperature TI and output temperature To,
b) determining a nominal heat QNOM corresponding to nominal operating conditions of the pre-catalyst,
c) determining an index I, representative of the state of wear of the pre-catalyst, as a function of this real heat QR and nominal heat QNOM.